Thermoplastic hot melt adhesives of the prior art typically comprise a thermoplastic polymer, a tackifier, a wax or plasticizing oil, and other optional ingredients. In large part, hot melt adhesive development in recent years has focused on the thermoplastic polymer used as the adhesive base material. Many inventions are based on finding a new property in a new thermoplastic polymer/adhesive composition or finding a new use for a known adhesive. A large variety of literature is directed to various thermoplastic polymers such as ethylene/vinyl acetate, ethylene methacrylate, atactic polypropylene, A--B--A block copolymers, A(BA).sub.n B block copolymers, etc. A brief selection of such literature includes:
Skeist, Handbook of Adhesives, Van Nostrand Reinhold Co. Inc. (1977).
Battersby et al., U.S. Pat. No. 3,318,977, teaches thermoplastic adhesives containing polyethylene, isobutylene rubber, tackifier resins, and ethylene-vinyl acetate copolymers.
Meeks et al., U.S. Pat. No. 3,971,883, teaches crosslinkable ethylene vinyl acetate copolymer resins in adherent laminates.
Taft et al., U.S. Pat. No. 3,982,051, teaches hot melt adhesive compositions containing ethylene/vinyl acetate and/or alkyl acrylate copolymers in hot melt carpet backing adhesives.
Boggs, U.S. Pat. No. 4,299,930, teaches hot melt adhesives containing modified polyethylenes and ethylene vinyl acetate copolymer and other materials.
Eastman, U.S. Pat. No. 4,293,473, teaches polyvinyl alcohol or ethylene vinyl alcohol copolymers in crystalline solvent based systems for bonding cellulosics, spun bonded polyolefins, aluminum foil, and other substrates.
Flanagan et al., U.S. Pat. No. 4,345,349, teaches a combination of an A--B--A block copolymer and ethylene vinyl acetate polymer and other standard hot melt ingredients to form an adhesive for book binding.
Tancrede et al., U.S. Pat. No. 4,497,936, teaches ethylene-vinyl acetate copolymers in combination with olefin rubbers in hot melt adhesives.
The adhesives disclosed in the typical prior art publications are adhesives that are applied as a melt liquid, and when cooled to ambient temperature become a solid. Once cooled, the adhesive mass attains relatively stable properties such as peel strength, shear strength, cold flow, or storage modulus, also known as G'. In other words, upon cooling, the adhesive formulation rapidly reaches a near equilibrium condition with respect to its physical state. As a result, each physical property rapidly attains the vast majority of its values immediately when cooled. We will refer to this state immediately after a hot melt adhesive cools and attains near equilibrium properties as the "ambient state" of the hot melt adhesive.
In the conventional production of prior art work pieces or articles, conventional hot melt adhesives are typically extruded at elevated temperature directly onto a bond site. The second substrate must be combined with the adhesive relatively quickly while the adhesive is still molten or liquid, or the adhesive will cool, harden, and set, making it impossible for the melt to wet the second substrate to form a bond. When the molten adhesive is extruded in a form of hot molten bead onto a porous substrate, the adhesive can wet or soak into one or more porous substrate and adhere the substrate to another substrate by entrapping fibers in the cooled adhesive mass. In such hot melt technology, the molten adhesive retains sufficient heat such that the material can retain a low melt viscosity, low modulus, and can penetrate or wet components of a work piece to ensure secure bonding before cooling. As such adhesives cool, the properties rapidly reach a final ambient state and, once the final values are reached, have, for the most part, stable values for that adhesive blend at ambient temperatures. However, such adhesives have the drawback that bonds must be made while the adhesive is at an elevated temperature to insure proper bonding, which prevents their use on heat sensitive substrates.
Pressure sensitive adhesives have been developed based on thermoplastic elastomers and a large variety of patents are directed to elastomeric based adhesives. A brief selection of such patents include:
Collins et al, U.S. Pat. No. 4,136,699, teaches a disposable article using a hot melt pressure-sensitive adhesive as a positioning or a construction material. Such adhesive is typically extruded to high temperature onto materials of construction during manufacture.
Chen et al, U.S. Pat. No. 4,460,364, teaches hot melt pressure-sensitive adhesives used in the manufacture of sanitary products.
Schmidt Jr., et al, U.S. Pat. No. 4,526,577, teaches the use of styrene-butadiene-styrene block copolymers in the manufacture of disposable laminates using multi-line extrusion adhesive technology.
Puletti et al, U.S. Pat. No. 4,627,847, also teaches the use of hot melt adhesives and disposable article construction.
Elastomeric based products have been developed which exhibit the property of pressure sensitivity. These types of products can form a bond after cooling, but generally do not cold flow after the adhesive reaches the ambient state. These elastomeric based pressure sensitive products are unable to form mechanical bonds or physically entrap fibers of a porous substrate after they are cooled and reach their ambient state. They tend to form surface bonds from the pressure sensitive nature of the adhesive.
Adhesives having properties that vary after application and cooling would offer significant advantages. In certain applications, initial bond strength is important, while after a time, the bond strength is preferably reduced. In other applications, fluidity is important while, after a time, the compositions are preferably in solid form. In still other applications, initial bond strength is not critical, while a high final bond strength is critical.
Accordingly, a substantial need exists in the industry for an adhesive that can have a controlled change in important properties after application. In other words, a need exists for an adhesive that does not reach a final equilibrium value for one or more physical properties until well after application and cooling.
In one aspect, a controlled change in modulus can provide important advantages. After application and cooling, the modulus of the adhesive is at an intermediate state between the modulus of the molten mass and the final modulus and is significantly below the modulus of typical comparable prior art hot melt adhesives. Such an adhesive with a controlled change or increase in modulus (G') can initially obtain significant cold flow after application that aids in construction of a variety of work pieces. Such cold flow can cause the adhesive to flow into a fabric to cause the physical entrapment of the materials of construction in the adhesive mass resulting in the formation of a bond of high integrity when the adhesive reaches its modulus potential. Such cold flow can also cause rapid and enhanced surface wetting of a nonporous surface producing enhanced bonding when fiber entrapment is not involved. Further, the bond can significantly resist the effects of a number of debonding mechanisms, including the presence of moisture or other compositions that can reduce or eliminate bond strength. After a time, the modulus increases to a final high equilibrium value forming strong cohesive bonds.
In recent years, increasing attention has been directed to the development of hot melt adhesives that can be sprayed onto the work piece or substrate during a manufacturing regimen. The use of spray-on adhesives has been found to increase productivity. Conventional spray-on adhesives are sprayed from a plurality of narrow orifices in a form of a fiber, a thread, a filament, or a plurality thereof, having a substantially circular cross-section with a diameter of 0.01"to 0.04". The spray-on adhesive fiber has substantial surface area in comparison to the mass or volume of the fiber. As a result, the sprayed adhesive fiber cools very rapidly upon contact with the ambient atmosphere. However, the spray-on adhesive, even if it retains some residual heat, attains the ambient temperature very quickly upon contact with the work piece. By ambient temperature, we mean the temperature of the surrounding atmosphere and the temperature of the work piece in the construction locale. This is in sharp contrast to extruded hot melt adhesives that retain significant amounts of heat for a period after application. Most conventional spray-on adhesives require a heated air flow at a temperature exceeding 250.degree. F., which is above the T.sub.g of styrene in the block copolymers used in the adhesive. Such a temperature is required to keep the adhesive molten until applied. In these spray-on adhesive applications, the temperature of the work piece and the manufacturing locus (not including the application equipment) are typically not substantially different. Conventional spray-on adhesives, after their application of work pieces, typically form a solid mesh or a web which is the result of the pattern in which the spray-on adhesive is applied to the substrate. An overlapping application pattern, as it is directed onto a moving web, typically takes the form of overlapping circles or ovals of adhesives that form a continuous adhesive strip or layer.
In another aspect, an adhesive with controlled bond strength can be important in a pallitizing adhesive and a carton-sealing adhesive which can have easily opened bonding. In such applications, initial peel and shear strength are important to secure the components in place. After assembly, the adhesive bond needs only sufficient shear strength to maintain the integrity of the assembly. At a use locus, the carton adhesive and the palletizing adhesive preferably have low peel strength permitting easy opening of the carton or easy disassembly of the pallet.